Over bore displacment

Sam Green · #1 02-16-2010, 05:32 AM

In this thread, http://www.fourwheelforum.com/showthread.php?t=88 a poster asked what was the displacment after a rebore/overbore.

For anyone intrested, here are the figures.

STD, new 49mm = 154.65cc.
Size # 1 49.25 = 156.23cc.
Size # 2 49.50 = 157.82cc.
Size # 3 49.75 = 159.42cc.
Size # 4 50mm = 161.02cc.

Each step up in capacity also increases the compression and results in a small bhp gain. I doubt you would be able to feel it but going from standard to max bore is noticable.

Sam.

aaron7 · #2 02-18-2010, 05:37 AM

I love seeing ads on ebay or craigslist for bikes that have been bored out .040 or .060 and they adamantly believe the displacement is WAY bigger.

Like a 49cc with an overbore being cited as a 70cc... and when you tell them that would require a new cylinder and such, they won't hear it haha (and yes I know there are big bore kits, but I'm just talking overbores)

Sam Green · #3 02-18-2010, 07:18 AM

I know what you mean. The only way to get a big increase from a minimal bore is on a motor with a stroke 5 times longer than the bore, the motor would fire once every 364.8 yards.

Sam.

65Honda · #4 03-08-2010, 06:11 PM

Okay a question from a car guy relatively new to this hobbie - I measured the pistons on my "dream" and they measure 1.965" (sorry I am stuck on imperial measurement) The top of the piston is marked 100 Can someone explain this?

Jim

hahnda · #5 03-08-2010, 08:01 PM

Quote:

Originally Posted by 65Honda

Okay a question from a car guy relatively new to this hobbie - I measured the pistons on my "dream" and they measure 1.965" (sorry I am stuck on imperial measurement) The top of the piston is marked 100 Can someone explain this?

Jim

My guess is that you are at 1.00mm over stock bore.

65Honda · #6 03-09-2010, 05:12 PM

hmm, Just measured the cylinders and they are at 1.969" which isn't too bad I guess. The size seems odd to me but like I said I am used to measuring in inches/thousands not mm's. The only reason I am considering changing the pistons is some minor "dings" on the top. The top ring was jammed when I pulled the engine apart but I managed to clean the goove up with a broken ring and gentle work with a precision file. Anyone have experience with doing this?

hahnda · #7 03-09-2010, 05:21 PM

Well one thing you might have against you is that I don't know if there are any CA95 pistons and rings that are in an overbore more than +1.00mm. You could possibly use CA/CB 160 pistons.

Were the pistons stuck or just the ring on the pistons that was stuck?

If it was only the stuck ring I would reuse the pistons and maybe even reuse the rings if I was feeling cheap. I would just make sure that the rings don't bind in any of the grooves.

65Honda · #8 03-10-2010, 02:12 PM

I have new rings that I have installed on the piston. The fit is okay and it should work okay. Not sure what the tolerance is but .004 doesn't seem like too much clearance between the piston and cylinder???

Sam Green · #9 03-17-2010, 02:20 AM

Quote:

Originally Posted by 65Honda

I have new rings that I have installed on the piston. The fit is okay and it should work okay. Not sure what the tolerance is but .004 doesn't seem like too much clearance between the piston and cylinder???

.004" is maximum wear, cast pistons in most Hondas are at .0012 with maximum at .002.
I run .002 in my race bikes and that's set up ready to race without any running in period.

Sam.

65Honda · #10 03-18-2010, 06:01 PM

So Sam, sounds like I am max oversize with my cylinder. What do you think the best solution? = go with a new set of 50mm pistons? Hone the cylinders to fit?

Smithers · #11 03-18-2010, 11:14 PM

A friend of mine (old racer tinkerer) built one of these engines and made his own crank and did all sorts of crazy stuff to it. One of these days when I visit him I'll get his details and some pictures. It's in pieces in a box and it's very heavily modified.

Sam Green · #12 03-20-2010, 01:16 AM

Quote:

Originally Posted by 65Honda

So Sam, sounds like I am max oversize with my cylinder. What do you think the best solution? = go with a new set of 50mm pistons? Hone the cylinders to fit?

Jim, the pistons you have are already that size, I tried to convert the inch figure that you gave and if I remember (it was a few days ago) you cylinder is almost at its limit for your piston (yours is the maximum over size.
You might want to get another block and start again with a smaller oversize.

Sam.

Sam Green · #13 03-22-2010, 04:25 AM

Quote:

Originally Posted by Smithers

A friend of mine (old racer tinkerer) built one of these engines and made his own crank and did all sorts of crazy stuff to it. One of these days when I visit him I'll get his details and some pictures. It's in pieces in a box and it's very heavily modified.

I'd sure like to see what he has done Ryan, pics are allways nice, everyone likes pictures

Sam.